This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The Reaction Center (RC), an integral membrane protein, undergoes light induced electron transfer reactions which are the primary steps in converting light into chemical energy. Upon excitation by light, electron transfer occurs from the excited state of a bacteriochlorophyll dimer (D) through intermediate acceptor molecules to two quinone molecules, QA and QB. The RC complexes with another pigment-containing integral membrane protein, the light-harvesting complex (LH1) which gathers and transfers light energy to the RC resulting in photo-oxidized D. Subsequently, a 14-kilodalton water soluble-cytochrome c2 transiently binds to the RC and re-reduces D. Our ongoing work at SSRL will focus on three structure-function aspects of the RC: 1). Determination of the structures of RC-metal complexes and mutants that affect proton and electron transfer events in the RC. 2). Determination of the structures of intermediate states involved in the charge separation. 3). Determination of the structure of the RC complexed with partner proteins: a). RC and light harvesting protein (LH1), and b). Mutants that affect the binding and electron transfer between RC with cyt c2.